Brightness, Contrast, Saturation, and Sharpness

Background

At first glance, it might
seem that doing an article on the four most common image controls would
be a waste of time. After all, brightness, contrast, saturation, and sharpness
are often thought to be the simplest controls as they've been around as
long as the color TV. People often overlook the fact that all
four are related, however, and changing any one of them can change
the other three. Do you know how they are related and how you are
changing the balance of brightness, contrast, saturation, and sharpness by only
changing one of the three parameters? Let's take a look.

Brightness

Brightness is generally thought to be
the simplest in concept. Just make the image brighter or darker by
a specified amount, right? First we must distinguish between true
brightness and something else called "gamma". Increasing gamma by
moving a mid-tone slider on a histogram is not the same as increasing
brightness. Increasing Gamma/mid-tones can make an image look
brighter, but it is non-linear in that it only increases brightness of
the shadows and mid-tones in an image without affecting the highlights.
Traditional brightness on the other hand, simply brightens the entire
image from the shadows to the highlights equally. Let's see what
happens when we add some brightness to an image. The following
test image is designed to bring out some of the effects we will refer to
in this article.

Fig
1: Increase Brightness

In figure 1 above, we have increased
brightness on the right half of both the B/W and color images. In
this case, we didn't increase brightness enough to clip the highlights
(brightest colors) so we've only affected brightness here.

Fig
2: Extreme Brightness

If we had made a more drastic change
such as the one shown in figure 2 where we added even more brightness,
we may have clipped the white/red spokes in the wheel which would have
affected contrast, saturation, and sharpness! In the extreme case
shown in figure 2 above, we have added so much brightness that the
shadows have "caught up" to the highlights because they are already as
bright as they can get. Now we have reduced saturation, reduced
contrast, and reduced sharpness as a result. The same effect can
be seen if we had reduced brightness to the point that the shadows had
nowhere else to go and the highlights started catching up to the
shadows. Depending on how close your shadows/highlights are to
their endpoints already, you don't need an extreme change in brightness
to affect the other parameters either. When increasing brightness,
you may find that you lose some contrast on the brightest details in the
image while the rest of the image has the same contrast as before.
Again, this is due to the clipping that is caused in the highlights.

Contrast

Contrast is defined as the separation
between the darkest and brightest areas of the image. Increase
contrast and you increase the separation between dark and bright,
making shadows darker and highlights brighter. Decrease contrast
and you bring the shadows up and the highlights down to make them closer
to one another. Adding contrast usually adds "pop" and makes an
image look more vibrant while decreasing contrast can make an image look
duller. Here is an example where we add some contrast.

Fig
3: Increase Contrast

In figure 3, we have added contrast
to the right half of both images. As you can see, the white/red
spokes have gotten brighter while the background has gotten darker.
This causes the image to look more defined. By making the
highlights brighter, however, we've also increased the brightness of the
spokes, causing the image to appear brighter since the spokes are the
main focus of the image. On the red image, increasing the
brightness of the spokes has also increase saturation (defined below).
Finally, sharpness has also been increased on both images (also defined
below). Here, we have increased brightness, contrast, saturation,
and sharpness simply by adding contrast! Note that not all areas
of the image will be affected equally and a lot depends on the content
of the image itself. Saturation effects, for example, will be less
noticeable in images that don't show bright colors because there is very
little saturation to begin with. As an extreme example, take a
look at the B/W image above. Since B/W images have zero saturation
by definition, changing contrast cannot change saturation in B/W (gray)
areas of your image.

Saturation

Saturation is similar to contrast,
however instead of increasing the separation between shadows and
highlights, we increase the separation between colors. An example
showing increased saturation would show the same effect as figure 3
above for the red image but the B/W image would not change at all because
B/W or gray detail has no saturation. As a result, an increase in
saturation results in an increase in contrast, brightness, and sharpness
on the red image as in figure 3 and no change to the B/W image.
Again, a change in saturation normally has a more noticeable effect on
vibrant colors and less on dull colors or colors that are almost
neutral. This is because to change saturation, there must be some
color saturation to work with in the first place.

Sharpness

Sharpness can be defined as edge
contrast, that is, the contrast along edges in a photo. When we
increase sharpness, we increase the contrast only along/near edges in
the photo while leaving smooth areas of the image alone. Let's
take a look at an example with increased sharpness.

Fig
4: Increase Sharpness

The right half of the above two
images has been sharpened using unsharp mask. By only sharpening
the edges, we've actually created several different effects in the above
image. Near the outer edge of the spokes, where the spokes are
thicker, they simply look sharper without looking brighter or more
contrasty. As we approach the center of the wheel, however, where
the spokes get very thin, our edge contrast enhancement has actually
caused the center of the wheel to get brighter, more contrasty, and more
saturated (on the red photo). This is due to the fact that most of the data near
the center is edge data so the effect increases in that area.
Here, we see that increasing sharpness can cause the appearance of
increase saturation, contrast, and brightness in areas of the image that
contain fine detail where other areas (areas with broader detail) seem
less affected except for the added sharpness.

Different effects for
different parts of an image

The overall effect of brightness,
contrast, saturation, and sharpness will vary depending on the content
in each photo. Consider increasing contrast as an example.
Increasing contrast makes shadows darker and highlights brighter.
If we increase contrast on an image where most of the detail in the
photo is very bright, say an overexposed sunset, we may actually end up
with less contrast! Why? Because there are no
(or minimal) shadows in the photo so separating the shadows and
highlights in an image that only contains highlights will just compress
the highlights, making them less contrasty. Similarly, taking a
soft focus shot and increasing saturation may cause bright/vivid colors
to appear sharper than gray or near gray detail and that may
cause an unwanted change in overall balance of the photo. As an
example, increasing saturation on a shot of a cricket sitting on a red
rose petal may increase the sharpness of the red rose petal, taking
focus off the less colorful subject (the cricket) because it will be
less affected by the change in saturation. The end result may be
that the rose petal now looks sharper than the cricket, making the
cricket appear to be out of focus, all because you increased saturation. Being able to control
these linked effects when using simple controls like brightness,
contrast, saturation, and sharpness is a bit of an art, but
understanding why we sometimes get unexpected results is
half the battle!

Summary

While brightness, contrast,
saturation, and sharpness may appear to be the simplest of image
controls on the surface and may appear to be mutually exclusive controls, they are
related and intertwined in such a way that changing any one of them can
create quite complex effects in your photos. Understanding how
they are related can be a big step in understanding how to use them and
more importantly when to use them. Before adding or
reducing brightness, contrast, saturation, or sharpness, think about
this article and ask yourself what you are really trying to accomplish.
Hopefully this article will help you pick the right control or the right
situation.